They emerge from the tunnel cut deep into a soggy Welsh hillside caked in black dust and mud. Backs are aching and ageing limbs are heavy but the men's faces are beaming.

Coal is already being hauled out of Unity mine in the Vale of Neath, south Wales, and by March the colliery will be in full swing, producing a million tonnes of high quality coal a year.

Business leaders, politicians and the miners themselves believe the reopening of Unity heralds the start of a rebirth in Welsh deep coal mining.

When Welsh coal mining was at its height in the 1920s, around 275,000 workers were employed in 600 large-scale pits, but by last year only one major deep mine was left - and it faces closure this year or next.

However, concern about the security of Britain's energy supply and new developments in clean coal technology, which allow harmful emissions to be reduced, are prompting companies like Horizon to look at reopening mines which closed down though they still had large reserves, and opening new ones.

Surveyors at Unity believe they have 89m tonnes of coal to go at and a business plan has been produced to keep the mine going for more than 25 years.

Horizon Mining, the company behind Unity, revealed to the Guardian yesterday that it is planning to reopen four more mines in south Wales and has bought up land with the intention of building at least three small-scale "clean coal" power stations.

A second company has reopened a mine across the road from Unity at Aberpergwm.

There are also ambitious plans afoot, which are being encouraged by the Welsh assembly and Whitehall, to develop a new deep mine at Margam, near Port Talbot, south Wales. If the scheme goes ahead, it would be the first deep coal mine to open in Britain in 30 years.

The Unity miners, many of whom thought they would never go underground again, are delighted.

"It's great to be back," said mine manager Tony Roberts, who was working in a factory when he got the call to return to the coalface. "Mining is in the blood, in the genes of people round here. There's nothing like the satisfaction of getting a tough job done. Miners never say that something's impossible - they just get on and do it. You miss the camaraderie when you're away from it. You rely on your mates to keep you safe and you keep them safe. That creates an unbreakable bond. And you have a laugh. The craic is just brilliant."

For the first time since the industry was run down in the 1980s and 1990s, hopes are growing that the political will to revive the mines that shaped the landscape and people of south Wales is there again.

Peter Hain, secretary of state for Wales, told the Guardian he was "excited" by the prospect of a renaissance in south Wales and across Britain. He said: "Coalfield communities in Britain were decimated by the collapse of mining, and its return would provide a much-needed boost."

Mr Hain added: "Although coal has historically been the dirtiest of fuels, new technologies now hold out the prospect of reducing coal emissions to a level comparable to gas. And with carbon capture, we may even eliminate emissions altogether."

The Welsh assembly energy minister, Andrew Davies, said: "Most people in Wales think of coal as part of our history. But it is, in fact, a fuel of the future."

All good news for the men at Unity. The gang preparing the mine are veterans, a dad's army of miners, some of whom spent more than 40 years underground before the closures forced them into other jobs. They became driving instructors, garage mechanics, factory workers until they got the call to help kickstart Unity.

Welder Barry Jones, 69, joked that he was only there to earn the money so that his funeral cortege can be led by four horses. "No, really, it's great that this is happening," he said. "The coal is there and the need for it is there. It's got to make sense to get it out."

Keith, 55, became a support worker for people with mental health problems after his last mine closed. "I jumped at the chance to come back," he said. "My father was a miner, my grandfather was a miner. It's what I know and love best."

The reopening of Unity and others will also provide a much-needed jobs boost in this deprived area. When it advertised for workers, more than 400 applied for the first 70 posts.

But Vernon Watkins, 60, warned that the government needed to make sure training programmes were in place for youngsters if the industry was to make a recovery. Nobody is under the illusion that the new mines will employ many hundreds of men as they used to but they can help cut local job queues.

He said: "We've got men here who can do the job now but we aren't going to be along for much longer. We need the youngsters now so we can pass on our experience to them.

"It's meaningful employment. It's not like working in McDonald's."

Britain's collieries

· British coal producers, most of them in England, meet only about a third of UK demand. The rest is imported from countries including Russia, South Africa, Australia, Indonesia and Colombia. In 2005, 44m tonnes of coal were imported.

· Coal-fired power stations account for more than 80% of total coal use. They produce a third of the UK's electricity. High gas prices in 2005-06 caused this to rise to a half.

· The international trade in fossil fuel is expected to double by 2030, thanks largely to the growth of industry in China and India

· In 1995-96 more than 50m tonnes of coal were produced from 83 deep and 122 surface mines in the UK. By 2005-6 this had dropped to 20m tonnes from 13 deep and 31 surface mines. There is estimated to be more than 500m tonnes of coal still available in the UK.

· Most British working mines are in England. The last of the great Welsh deep mines, Tower colliery, Hirwaun, saved by a workers' buyout in 1995, is expected to cease mining this year or next. It hopes to reinvent itself as a school of mining and a global expert in clean coal technology.